The present invention relates to a CSMA type communication system and, more particularly, to a CSMA type communication system which is capable of effecting a high speed transmission of digital data among a plurality of units.
As a conventional communication system for transmitting digital data among a plurality of units which operate asynchronously with one another, there has been available a system known as the CSMA (Carrier Sense Multiple Access) type communication system. Such a system is disclosed in, for example, Japanese Patent Application Kokai No. Sho 56(1981)-169,452.
In a CSMA type transmission system, any number of units connected to a bus may monitor a signal on the bus and, in the absence of the signal on the bus a transmission frame can be forwarded out to the bus at any time. The transmission frame thus sent to the bus is generally formed by such fields as a master address field, a slave address field, a control code field and a data field. These fields represent an address data of the unit outputting the transmission frame, an address data of a destination unit, a kind of transmission data or a direction of data transfer and a transmission data, respectively. The unit which has detected a transmission frame on the bus receives a master address field and a slave address field in synchronization with the synchronizing signal contained in the transmission frame. When the value of the slave address field received coincides with that of inherent address data which was assigned to the received unit, the unit receives further a control code field. If the code of this control code indicates transfer of data from the master unit to the slave unit, the unit receives the data field to follow. On the contrary, if the control code indicates transfer of data from the slave unit to the master unit, the data is sent to the bus at the data field in synchronization with the synchronizing signal.
Thus, in the conventional system, the master unit monitoring a signal on the bus sends a transmission frame and the slave unit receives such transmission frame. Further, the data field of the transmission frame is such that either the master unit or the slave unit may fill the data to be transmitted.
In the conventional CSMA type transmission system as explained above, the communication with respect to one frame is made only between the master side unit and the slave side unit, so that it is necessary for the system to have as many transmission frames as the number of the units in order to receive the digital data through the bus among a number of units connected to the bus. This results in a disadvantage in that the time required for transmission is considerably longer than that required in the communication between the two stations.